[SPARK-7895] [STREAMING] [EXAMPLES] Move Kafka examples from scala-2.10/src to src

Since `spark-streaming-kafka` now is published for both Scala 2.10 and 2.11, we can move `KafkaWordCount` and `DirectKafkaWordCount` from `examples/scala-2.10/src/` to `examples/src/` so that they will appear in `spark-examples-***-jar` for Scala 2.11.

Author: zsxwing <zsxwing@gmail.com>

Closes #6436 from zsxwing/SPARK-7895 and squashes the following commits:

c6052f1 [zsxwing] Update examples/pom.xml
0bcfa87 [zsxwing] Fix the sleep time
b9d1256 [zsxwing] Move Kafka examples from scala-2.10/src to src